Conventionally, variable speed motor input drives, are utilized to drive a primary stage ball screw and move a ball nut the greater proportion of the actuator""s linear stroke at a relatively high speed, and then are cushioned to abruptly decelerate during the end portion of the stroke. This is normal practice for multiple stage actuators, such as the type shown in the present assignee""s U.S. Pat. No. 3,762,227, in which there is a multiple extension of the actuator components in stages, but no provision for providing automatic xe2x80x9cend of strokexe2x80x9d speed reduction.
It is desirable that the actuator be improved so that the drive motor utilized can provide a constant speed input. The invention provides an elongate, revolvable, axially restrained ball screw and accompanying ball nut having thread forming helical land and groove portions which provide a raceway between them for load bearing balls, wherein the ball nut travels a major portion of the stroke of the multiple extend linear actuator. The ball nut is capable of rotation, as well as translation along the ball screw.
An actuator nut, with internal threads, is mounted on the ball nut, which is provided with matching external threads. The hand of the threads of the actuator nut is the same as the hand of the threads of the ball nut, but the actual nut threads have a shorter lead by at least a factor of two. The ball screw is provided with a resilient energy dissipating stop which extends into the path of the ball nut to first decelerate the travel of the ball nut and then to effect a coupling of the screw shaft and ball nut so that they begin rotating as an integrated assembly at the constant speed of the motor. This causes the actuator nut to extend, but at a significantly reduced speed. The reversing stroke of the actuator is similar in that the major portion of the reversing travel is created by the ball nut traveling along the ball screw, the ball screw then being stopped and rigidly coupled with the ball screw to initiate travel of the actuator nut in the reverse direction.
It is a prime object of the present invention to be able to to drive the ball screw at a constant speed during both the forwarding and reversing travel of the linear actuator.
It is a further object of the invention to provide an actuator assembly which is so constructed as to provide a linear deceleration of the ball nut and to effect a coupling of the ball nut to the screw shaft so that the ball screw and the ball nut rotate as a rigid couple which moves the actuate member being actuated at a much reduced speed at the end of the actuator stroke.
Another object of the invention is to provide a relatively durable, maintenance free, multiple stage actuator system which operates smoothly and reliably to effect an actuation of the system to which the actuator nut is connected.
Another object of the invention is to provide an economically available actuator and drive which does not impose undue loads on the system which it operates.
Other objects and advantages of the invention will become apparent with reference to the accompanying drawings and the accompanying descriptive matter.